Typical air purifiers use electrode discharge ultraviolet lamps; in some cases, the ultraviolet light is relied upon to wholly or partially destroy microorganisms; in other cases, the ultraviolet light activates a photocatalyst on the surface of a packed bed of pellets or of a structure which is permeable by the air flowing therethrough. Similar apparatus may be used in chemical processes in which photocatalytic oxidation of air or water streams is performed either to remediate (i.e., sterilize) or otherwise condition the stream.
Because of the short lamp life, air-purifier designs must be made suitable for lamp changeover. Such designs are unnecessarily large in volume, require a large number of ultraviolet lamps and photocatalyst elements (when used). The efficiency of electrode ultraviolet lamps (such as mercury lamps) is less than 30%; that is, less than 30% of the electrical input is converted to ultraviolet radiation. Lamps of this sort are utilized in U.S. Pat. No. 6,280,686 and patents referred to therein.
Consider, as an example, current, standard practice employed in commercial photocatalytic treatment of process streams, such as air-purification of contaminated air in occupied space of buildings. An effective design requires the bringing together, in space and time, of the ultraviolet photon, the photocatalyst surface element, and the process stream (e.g., the contaminants in the air in the air-purification example). Present design practice fixes the photocatalyst in space and places the ultraviolet irradiating source externally to the photocatalyst. Consequently, photocatalyst surface elements are rendered non-uniformly irradiated, dimly irradiated, and non-irradiated. This practice necessarily imposes a limit on the degrees of freedom available to the design.
As an illustrative application example, present commercial photocatalytic air purifiers all use electrode discharge ultraviolet lamps and a photocatalyst element that is configured as a packed bed of photocatalyst pellets or as a porous monolith support (i.e., honeycomb, reticulate foam, screen, woven or unwoven fiber, etc.) having a photocatalyst coating. The primary deficiencies of this design are twofold: First, because the source of ultraviolet radiation is external to the photocatalyst support and photocatalyst surface, that is, the source of ultraviolet radiation and the photocatalyst surface are necessarily separate in space, the illumination is inherently non-uniform, which results in dimly irradiated or non-irradiated surface elements, and consequently poor contaminant destruction. Because of this inherent deficiency, the design objective of delivery of the ultraviolet photons to the photocatalyst, while simultaneously achieving delivery of the contaminant (process stream) to a suitably activated photocatalyst, creates a difficult design problem. Second, fluid processing apparatus has heretofore been limited by the use of ultraviolet lamps which rely on an electric discharge between electrodes in order to sustain the creation of ultraviolet radiation. These devices suffer from the deposit of impurities resulting from heat concentration at the electrodes, which in turn inhibits electron emission and, therefore, UV photo emission. Lamp irradiation diminishes with time as the lamp ages and results in a useful lamp life that is less than about 8,000 hours; an inherent characteristic of electrode based ultraviolet lamps. These deficiencies result in air-purifier designs that are large in volume, require a large number of ultraviolet lamps and photocatalyst elements, and consume a large amount of electrical power. Although the air-purification example is used to illustrate shortcomings of external irradiation, those same deficiencies are inherent in all applications that have the ultraviolet radiation sources external to the photocatalyst.